John Chrysostom
Why, you ask, did the Spirit of the Lord catch Philip away? Because he had to go to other cities and preach. This was done so that he himself, later marveling at it, would not think what happened to him was a human work, but God's.
Here something worthy of wonder is revealed. What formerly happened rarely, and scarcely occurred at all, here takes place very often.
Moreover, this event was a certain foreshadowing that the apostles would also convert foreigners. A credible testimony could dispose others who learned of it to the same zeal.
For this reason the eunuch also went on his way “rejoicing.” If he had not come to know, he would not have rejoiced.
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But what, you ask, prevented him from learning all this well while sitting in the chariot, especially in the desert? The fact that it was not clear.
“When they came up out of the water, the angel of the Lord caught Philip away.” This was well done, so that it might be manifest that what happened was the work of God, and that the eunuch might not think Philip was an ordinary man.
“And he went on his way rejoicing.” The writer says this to show that he would have grieved had he known of Philip's removal. But from the great joy he received through the Spirit, he did not even notice what happened around him.